1. Technical Field of the Invention
The present invention relates to an image scanning apparatus for use in a copying machine or image reader, and more particularly, to an image scanning apparatus for exposing an image or reading an image by scanning an original when a scanning system which is driven in reciprocating motion by a motor is moved forward.
2. Description of Related Art
Nowadays, high-speed operation is more and more required for copying machines and image readers. It is thus desired to reduce the time of operation by the scanning system when a multiplicity of copies are produced from one original or when a multiplicity of originals are successively scanned. In order to satisfy the requirement, a motor which is a driving source is adapted to cause the scanning system to perform backward movement at high speed with full power when it is not engaged in a scanning operation. Accordingly, it is difficult to accurately stop the scanning system at its home position when it is moved backward.
In U.S. Pat. No. 4,561,771 and U.S. Pat. No. 4,568,171, there are disclosed original scanning apparatuses capable of solving the problem. In those apparatuses, there is provided a brake switch at a predetermined position in front of the home position of the scanning system, and when the scanning system has reached the position where the brake switch is actuated during backward movement of the scanning system, brake is applied to the motor with the action of the brake switch so as to stop the scanning system accurately at its home position. In this method, however, an extra switch with its wiring is necessary in addition to a switch which is arranged at a fixed position for detecting a scan start position of the scanning system thereby causing a rise in manufacturing cost.
Japanese Patent Publication TOKKAI SH055-163560 discloses a return control method by a software control for stopping the scanning system at its home position when it is operated for backward movement, wherein the scanning system is moved backward for a certain period of time and the amount of displacement occurred in the period is measured by which a brake is actuated with a driving force corresponding to the amount of the displacement to stop the scanning system. When such software control is applied, complication in mechanical structure can be avoided. However, the amount of movement of the scanning system is not measured directly from the time of operation, and it can not cope with a change in the amount of movement of the scanning system with respect to a change in load arising from changes in curvature of mirror and aged deterioration of bearing and adhesion of dust in the scanning system. Accordingly, the scanning system can not be stopped accurately at a predetermined position at all times.
Further, a motor has to be controlled to be driven at a constant speed since it drives a structure which is required to be moved at a fixed speed like the scanning system which scans original image in a copying machine. For controlling the speed of the motor, a method is adopted in a well-known conventional device, wherein the frequency voltage of an encoder pulse which synchronizes with the rotation of the motor is converted to compare with a standard voltage which corresponds to a desired speed and is amplified to make it as a signal for turning on electricity through a further pulse width modulation process. Another method is also adopted in which an encoder pulse which synchronizes with the rotation of the motor is inputted into a calculation means to calculate it for comparison in order to obtain a signal for turning on electricity which synchronizes with the encoder pulse for controlling the speed of the motor.
However, in these methods, the timing for applying electricity to the motor is synchronized with an encoder pulse synchronously generated with the rotation of the motor, and low-frequency noise is produced since the pulse width of the encoder pulse under maximum magnification becomes longer especially in a copying machine which is capable of changing magnification. Different pulse widths of the encoder pulse according to different magnifications offend the ear since low-frequency noise in different sounds are produced corresponding to the magnifications. Further, the speed of the motor can not be controlled closely because of long duration of period in applying electricity, and the constant speed control tends to become unstable.